Chalcogenide glasses are composed of two or more chalcogen elements normally selected from the small group including As, Ge, Sb, P, Te, Se, and S. When these elemental materials are heated and mixed in an oxygen-free environment, some very stable and simple glasses can result. One of the oldest chalcogenide glasses studied is the binary glass arsenic trisulfide, As2S3. This glass is deep red in color, and it is very stable. In the mid 1960s, this glass was drawn into the first IR fiber by N. S. Kapany. In his 1967 book on fiber optics, Kapany includes a discussion of As2S3 fibers and some of their potential applications. It was not until some 10 to 15 years later, however, that these materials were studied seriously as viable IR fiber candidates. The reticence to pursue these materials in the early days came in part from the toxic nature of some of the elements used in the glasses. Today they are a popular IR fiber material as they are readily drawn into fiber with a broadband IR transmission. While they are much more delicate in nature than the oxide glass fibers, they are finding many applications in chemical and temperature sensor systems and as IR image bundles.
Chalcogenide fibers fall into three categories: sulfide, selenide, and telluride. Within these categories, one usually finds that the binary and ternary glasses are excellent choices for fiberization. That is, unlike the fluoride glasses where it is commonplace to have five or more components, most chalcogenide glasses have only two or three elemental components. In general, these glasses have softening temperatures comparable to fluoride glass. They are very stable, durable, and largely insensitive to moisture. A distinctive difference between these glasses and the other IR fiber glasses is that they do not transmit well in the visible region and their refractive indices are quite high. Additionally, most of the chalcogenide glasses, except for As2S3, have a rather large value of dn/dT. This fact limits the laser power handling capability of the fibers.
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